Jesús Arnau

Performance study of multiuser interference mitigation schemes for hybrid broadband multibeam satellite architectures

As the demand for higher throughput satellites increases, multibeam architectures with smaller beam spots are becoming common place. If the same frequency is strongly reused, the resulting interference when serving simultaneously many users requires some sort of pre or post-cancelation process. This article focuses on precoding and multiuser detection schemes for multibeam satellites, comparing hybrid on-board on-ground beamforming techniques with fully ground-based beamforming. Both techniques rely on the exchange of radiating element signals between the satellite and the corresponding gateway but, in the latter case, the interference mitigation process acts on all the radiating signals instead of the user beams directly, with the corresponding extra degrees of freedom for those cases for which the number of radiating elements is higher than the number of user beams. The analysis carried out in this study has shown that the potential advantage of ground-based beamforming may exceed 20% of the total throughput.

Fractional pilot reuse in massive MIMO systems

Pilot contamination is known to be one of the main impairments for massive MIMO multi-cell communications. Inspired by the concept of fractional frequency reuse and by recent contributions on pilot reutilization among non-adjacent cells, we propose a new pilot allocation scheme to mitigate this effect. The key idea is to allow users in neighboring cells that are closest to their base stations to reuse the same pilot sequences. Focusing on the uplink, we obtain expressions for the overall spectral efficiency per cell for different linear combining techniques at the base station and use them to obtain both the optimal pilot reuse parameters and the optimal number of scheduled users. Numerical results show a remarkable improvement in terms of spectral efficiency with respect to the existing techniques.

Adaptive Transmission Techniques for Mobile Satellite Links

Adapting the transmission rate in an LMS channel is a challenging task because of the relatively fast time variations, of the long delays involved, and of the difficulty in mapping the parameters of a time-varying channel into communication performance. In this paper, we propose two strategies for dealing with these impairments, namely, multi-layer coding (MLC) in the forward link, and open-loop adaptation in the return link. Both strategies rely on physical-layer abstraction tools for predicting the link performance. We will show that, in both cases, it is possible to increase the average spectral efficiency while at the same time keeping the outage probability under a given threshold. To do so, the forward link strategy will rely on introducing some latency in the data stream by using retransmissions. The return link, on the other hand, will rely on a statistical characterization of a physical-layer abstraction measure.

Multiuser detection performance in multibeam satellite links under imperfect CSI

In multibeam satellite systems, there is a growing need for signal processing techniques able to mitigate the interference among beams, since they could enable a much more aggressive spectrum reuse. In this paper, we investigate the effect of the absence of perfect Channel State Information (CSI) at the receiver end of the multibeam satellite return link. Under the assumption of a large number of beams, random matrix theory tools are used to obtain closed-form expressions of the performance for a given channel matrix and different profiles of estimation errors.

Impact of LOS/NLOS propagation and path loss in ultra-dense cellular networks

Most prior work on performance analysis of ultradense cellular networks (UDNs) has considered standard power-law path loss models and non-line-of-sight (NLOS) propagation modeled by Rayleigh fading. The effect of line-of-sight (LOS) on coverage and throughput and its implication on network densification are still not fully understood. In this paper, we investigate the performance of UDNs when the signal propagation includes both LOS and NLOS components. Using a stochastic geometry based cellular network model, we derive expressions for the coverage probability, as well as tight approximations and upper bounds for both closest and strongest base station (BS) association. Our results show that under standard singular path loss model, LOS propagation increases the coverage, especially with nearest BS association. On the contrary, using dual slope path loss, LOS propagation is beneficial with closest BS association and detrimental for strongest BS association.

Performance analysis of multiuser detection for multibeam satellites under rain fading

Multibeam satellite systems are nowadays widely employed, and their use is expected to grow in the next decades. This has raised the interest for signal processing techniques able to mitigate the interference among beams, since they could enable a much more aggressive spectrum reuse. From an engineering point of view, the system design must consider as an option aggressive frequency reuse patterns, with the need to evaluate the performance of the corresponding interfering canceling techniques for the usual impairments and non-idealities of the satellite link. In this paper, we investigate the effect of a practical impairment of great importance, namely, the attenuation due to the rain, on the performance of a return link which performs MMSE interference canceling. Analytical expressions are derived for a number of performance measures based on the statistical characterization of the rain attenuation.

Performance of the Multibeam Satellite Return Link With Correlated Rain Attenuation

Rain attenuation is among the major impairments for satellite systems operating in the K-band and above. In this paper, we investigate the impact of spatially correlated rain attenuation on the performance of a multibeam satellite return link. For a comprehensive assessment, an analytical model for the antenna pattern that generates the beams is also proposed. We focus on the outage capacity of the link and obtain analytical approximations at high and low signal-to-noise ratio. The derived approximations provide insights into the effect of key system parameters such as the interuser distance, the satellite beam radius, or the rain intensity, and simulation results show that it fits tightly with the Monte Carlo results. Additionally, the derived expressions can be easily particularized for the single-user case, providing some novel insights.

Open Loop Adaptive Coding and Modulation for Mobile Satellite Return Links

In this work we have tested the applicability of open-loop Adaptive Coding and Modulation (ACM) in the return link of a BGAN-like mobile satellite link using, for improved performance monitoring, effective SNR metrics instead of conventional average SINR as Channel State Information (CSI). After testing the applicability of open-loop CSI, we carried out a performance test focusing on specific working conditions. Results will show that, for the scenario under study, the best performance is obtained in an ITS environment, reaching an improvement of up to 92% in terms of ASE, and 12% more availability.

Balancing closed and open loop CSI in mobile satellite link adaptation

We consider the problem of modulation and coding scheme selection in the return link of a mobile satellite system. We propose to use a weighted combination of both open loop and closed loop signal quality indicators to perform this selection. The combination weights are not selected by making any assumptions on the channel distribution; instead, they are dynamically adapted according to the ACK/NAK exchange between both ends. This adaptation procedure is obtained as a stochastic programming solution to an optimization problem. Numerical results will show the good performance of the proposed method compared to previous algorithms, and its robustness to environment changes.

Performance analysis of ultra-dense networks with elevated base stations

This paper analyzes the downlink performance of ultra-dense networks with elevated base stations (BSs). We consider a general dual-slope pathloss model with distance-dependent probability of line-of-sight (LOS) transmission between BSs and receivers. Specifically, we consider the scenario where each link may be obstructed by randomly placed buildings. Using tools from stochastic geometry, we show that both coverage probability and area spectral efficiency decay to zero as the BS density grows large. Interestingly, we show that the BS height alone has a detrimental effect on the system performance even when the standard single-slope pathloss model is adopted.